Materials with negative electron affinity or low electron affinity are known, which are generally of carbon with diamond structure. These materials have the great advantage of emitting electrons under weak extraction fields (of the order of 10 V/xcexcm). Since it is easy to obtain such fields on a planar thin film, it is no longer necessary to create tips in order to fabricate cathodes, and this facilitates the fabrication process. For example, in a tipped cathode it is essential to control the diameter of the holes in the extraction grid to within 0.1 xcexcm.
W. Zhu et al. have studied polycrystalline diamond deposits obtained by CVD (chemical vapour deposition) and have shown that the emission density increases significantly with the density of defects which the films contain. Certain deposition conditions make it possible to obtain layers exhibiting, for fields of the order of 30 V/xcexcm, current densities of 10 mA/cm2, i.e. a sufficient value for fabricating a screen with a luminosity of 300 cd/m2. However, the emissive properties of the films do not appear very uniform because they depend greatly on the surface roughness (of the order of the grain size ≈5 xcexcm) and the defect density. In field-emission screens whose cathodes are made of polycrystalline material, it is therefore found that the display is not uniform.
The invention makes it possible to solve this problem by proposing to make the cathodes of an information display screen from a low-electron-affinity material of amorphous or crystalline structure which exhibits a smooth surface condition. However, such cathodes cannot emit a strong electron flux (less than 1 mA/cm2, about 10xe2x88x925 A/cm2). In a matrix screen, for example of 1000xc3x971000 rows, the picture elements are in principle driven row by row. In order to solve the problem of low power emitted by each pixel (each cathode), it is proposed to associate, with each cathode, a switching device which sustains the drive of the cathode during a frame time, a frame time being the total time necessary for driving all the rows of a screen one after the other. Under these conditions, it can be assumed that the intensity emitted by a cathode integrated over a frame time is virtually equivalent to the power which would have been necessary in a row-by-row drive, multiplied by the number of rows. In other words, according to the invention, the low-electron affinity cathodes characterized by a low emission density ( less than 1 mA/cm2) can be used in a display screen so long as they are each combined with a drive circuit which sustains the current supply during a frame time, which makes it possible to have a current supply n times smaller than that which would have been necessary in a row-by-row drive, n being the number of rows of the screen.
The invention therefore relates to a drive system for a screen comprising at least one electron-emission picture element with low electron affinity, characterized in that it includes:
a set of cathodes arranged in rows and columns, and driven row by row;
a switching device associated with the cathode of each picture element and making it possible to connect the said cathode to a current source during a time necessary for the driving of all the rows and to regulate the current conduction of the corresponding picture element.